Liquid source materials for chemical vapor deposition (CVD) are becoming widely utilized, at least in part due the fact that in many circumstances CVD cannot be accomplished using compounds that are gases at ambient conditions. Liquid sources utilized in CVD include such sources as tetraethoxysilane (TEOS) used as a source of silicon to deposit silicon dioxide films, sources for use and deposit of titanium nitride (TiN) films by CVD, and sources for depositing metal oxides (for example, tantalum oxide, niobium oxide, aluminum oxide, titanium oxide), ferroelectric oxides, copper, and aluminum. Liquid sources used for doping by diffusion are typically organic sources, such as, for example, phosphorus oxychloride, phosphorus tribomide, phosphorus trichloride, and boron tribomide. Further, for depositing doped films by CVD (e.g., borophosphosilicate glass, borosilicate glass, phosphosilicate glass), common liquid sources include, for example, triethylborate, triethylphosphate, triethylphosphite, triisopropyl borate, trimethylborate, trimethylphosphate, and trimethylphosphite. The liquid precursors listed above are listed for illustration only and there are many other liquid precursors too numerous to list. For example, some additional liquid precursors, such as carboxylate complexes, are described in copending utility application, entitled "Method of Depositing Films on Semiconductor Devices" (Docket No. 150.00190101) filed on even date herewith and to which the present invention is equally applicable for vaporization and delivery thereof This copending application is incorporated herein by reference.
Liquid sources are so named because their vapor pressures are so low that they are liquids at room temperature. However, some materials, such as, boron trichloride, have fairly high vapor pressures and are only barely in the liquid state at room temperature. The lower a material's vapor pressure, the more difficult it is to deliver to a CVD reactor or processing chamber. The most commonly used liquid source, TEOS, has a low vapor pressure and many other liquid sources utilized for CVD have even lower vapor pressures. While TEOS can be delivered with existing bubbler technology where a carrier gas, typically nitrogen, is bubbled through the liquid to sweep some of the liquid source molecules into the processing chamber, other liquid precursors, such as precursors for deposition of metal oxide films, due to their lower vapor pressures cannot be delivered with sufficient reproducibility with such bubbler delivery systems, particularly in device applications with small dimensions.
Therefore, there is a need for improvement in conversion of liquid precursors to a vapor and delivery of such vaporized liquid precursors to wafer surfaces. As mentioned above, bubbler delivery systems can be utilized; however, such systems have the disadvantage of having the flow of the liquid precursor indirectly controlled via control of the carrier gas flow bubbled through the liquid precursor. Further, bubblers also have problems in delivering materials with very low vapor pressures which tend to condense or decompose near normal temperatures required for vaporization between the source of the liquid precursor and the processing chamber used for CVD.
Alternatives to conventional bubbler technology, include an approach wherein the liquid source material is heated and vapors are drawn off and controlled by a vapor mass flow controller. Further, another way is to transfer the liquid precursor using either a very precise metering pump or a liquid mass flow controller up to the point where it enters the reaction chamber. At that point, it can either be flash vaporized or injected directly into a mixing chamber and showerhead where it is vaporized. As described in the article entitled, "Metalorganic Chemical Vapor Deposition By Pulsed Liquid Injection Using an Ultrasonic Nozzle: Titanium Dioxide on Sapphire from Titanium (IV) Isopropoxide," by Versteeg, et al., Journal of the American Ceramic Society, Vol. 78, No.10, pgs. 2763-68 (1995) a metalorganic CVD process utilizes pulsed on/off liquid injection in conjunction with atomization by an ultrasonic, piezoelectrically driven nozzle to deliver such metalorganic precursors. The pulse injection is said to allow control of film deposition rates, as fme as monolayers per pulse. The ultrasonic nozzle provides a mist of droplets into the processing chamber of a reactor for reproducible vaporization of the liquid precursor. However, such a delivery system performs the vaporization in the processing chamber and thus this delivery system would not be adequate for precursors with only moderate volatility. Such a mist or microdroplets of precursors having only moderate volatility generated by the ultrasonic nozzle would not entirely vaporize prior to contacting the wafer surface in the processing chamber and the CVD film uniformity would not be adequate.
In current systems, where liquid precursors are delivered to a vaporizer using mist generation, vaporization is typically carried out by contact with heated surfaces and then a carrier gas is used to deliver the vaporized liquid precursor to the processing chamber. Such vaporizing devices for delivery systems suffer from the disadvantage of decomposition of the liquid precursors upon contact with the hot surfaces, or incomplete vaporization, which also yields inconsistent films grown under CVD conditions.
For the above reasons, there is a need in the art to provide highly reproducible vaporization of liquid CVD precursors. The present invention as described below improves upon the vaporization process and overcomes the problems described above and other problems which will become apparent to one skilled in the art from the description below.